Along with the computerization of offices in the 1980's came electronically controlled non-impact printers such as the ink-jet and laser printers. Drop-on-demand ink-jet printers can be piezo or thermal. With piezo ink-jet systems, ink droplets can be ejected by an oscillating piezo crystal. However, thermal ink-jet printers dominates the drop-on-demand office ink-jet market. With this system, rapid heating behind the ink nozzles cause a bubble of vapor to form in the ink. The resulting bubble expansion and ink ejection from the ink-jet printer cartridge causes printing to appear on the substrate.
At the present time, full-color ink-jet printers are more common than color laser printers and are much more economical. The main advantage of ink-jet printers over laser printers and other non-impact printing techniques include their low cost and simplicity. Thermal ink-jet systems are capable of dispensing ink rapidly and accurately. The technology of this and other ink-jet systems are discussed in the Chemistry and Technology of Printing and Imaging Systems, edited by P. Gregory, published by Chapman & Hall, 1996. Representative thermal ink-jet systems and cartridges are discussed in U.S. Pat. No. 4,500,895 to Buck et al., U.S. Pat. No. 4,513,298 to Scheu, and U.S. Pat. No. 4,794,409 to Cowger et al., which are all hereby incorporated by reference.
The technology associated with ink-jet printers has undergone many changes and improvements since they first appeared. Research has been conducted to ensure that the images produced are of consistent high quality. Thus, in one aspect, it can be important that the images be waterfast and do not smear, smudge, run, or the like, when exposed to chemical or mechanical abrasion. Non-smearing of an ink-jet printed image when portion of the image is highlighted with a colored marker is of particular interest, as it provides a simple means of judging smudge resistance. Oftentimes, an image produced by an ink-jet printer on paper is not satisfactorily fixed, and thus, can smear, blurring the printed image when subjected to highlighting. This type of image is not regarded as permanent.
Image permanence can be defined as transference of color from the substrate when the image printed thereon is subjected to chemical and mechanical abrasion. Highlighting is oftentimes the form of chemical and mechanical abrasion experienced. This transference of color is measured by optical density. More permanent images have lower milli-Optical Density (mOD) values.
Another desired feature of printed images is light fastness. As used herein, light fastness will mean that the images do not significantly fade over time when exposed to light. Light fastness is another measure of permanence as used herein. Light fastness can be measured by exposing a printed image to intense light in a light chamber (fadomer) and comparing print density before and after the exposure.
There have been many past attempts of improving the permanence of water-based ink-jet printing systems. Included among these attempts are U.S. Pat. No. 5,549,740 to Takahashi et al., U.S. Pat. No. 5,640,187 to Kashiwakazi et al., and U.S. Pat. No. 5,792,249 to Shirota et al. which utilizes an additional or “fifth” pen to apply a colorless fluid on to the substrate. As will be seen in the comparative testing, the mOD values for the images printed thereon are quite high.
Another highly efficient printing system in common use currently is the laser printing system. With a laser printer or copier, light from a laser beam is used to discharge areas of a photoreceptor to create an electrostatic image of the page to be printed. The image is created by a printer controller, a dedicated computer in the printer, and is passed to the print engine. The print engine transcribes an array of dots created by the printer controller into a printed image. The print engine can include a laser scanning assembly, photoreceptor, toner hopper, developer unit, corotrons, discharge lamp, fuser, paper transport, paper input feeders, and paper output trays.
The final stage of laser printing or copying is to fix the toner onto the paper. Toner is very fine plastic powder that is transferred from the photoreceptor to the paper. Once transferred from the photoreceptor, it lies on the paper in a very thin coating with nothing to hold it in place. In order to fix the toner to the paper, it is typically heated by passing between a pair of very hot rollers, so that the plastic melts around the fibers of the paper and is “fused” into place. The image is now fixed permanently onto the paper.
The fuser of a typical laser printer is of particular interest to the printing system of the present invention. In these systems, fusing or melting the polymeric resin in which the colorant is embedded converts the discrete toner particles into an amorphous film. This film becomes the permanent image that results in electrophotographic copy or laser printed copy. However, laser printer toners are typically incompatible with water.
Another printing technology that is inherently more permanent than water-based ink-jet inks are hot-melt inks. These materials are solid at room temperature and are similar to wax crayons. The colorants used in these materials are solvent dyes that are soluble in the ink vehicle or pigment dispersions. Like laser toners, these materials are typically incompatible with the inks used in ink-jet printing.
U.S. Pat. Nos. 5,817,169 and 5,698,017, both to Sacripante et al., disclose hot melt ink compositions which use oxazoline as a vehicle used for a colorant in a nonaqueous, hot melt, ink-jet ink. One of the advantages of this technology is the waxy nature of the hot melt ink creates images that are more waterfast and may be successfully utilized on plain papers. This technology is in contrast with the instant invention, which can utilize an ordinary aqueous four-color ink pen set.
Accordingly, a need remains for a printing system using water-based ink-jet technology, which produces permanent images, and can be printed on plain paper substrates. In addition to plain paper substrates, these permanent images will be consistent and stable with respect to a variety of other substrates. An ideal situation would be combining the convenience and safety of aqueous ink-jet inks with the permanence of electrophotographic copies. The present invention satisfies this in a unique manner, which is described herein.